File 58343f29-x86-emul-correct-the-IDT-entry-calculation-in-inject_swint.patch of Package xen.4218

Subject: x86/emul: correct the IDT entry calculation in inject_swint()
From: Andrew Cooper andrew.cooper3@citrix.com Tue Nov 22 14:29:26 2016 +0100
Date: Tue Nov 22 14:29:26 2016 +0100:
Git: 34fbae790c98966903d7f97c6f46622ea842ab55

The logic, as introduced in c/s 36ebf14ebe "x86/emulate: support for emulating
software event injection" is buggy.  The size of an IDT entry depends on long
mode being active, not the width of the code segment currently in use.

In particular, this means that a compatibility code segment which hits
emulation for software event injection will end up using an incorrect offset
in the IDT for DPL/Presence checking.  In practice, this only occurs on old
AMD hardware lacking NRip support; all newer AMD hardware, and all Intel
hardware bypass this path in the emulator.

While here, fix a minor issue with reading the IDT entry.  The return value
from ops->read() wasn't checked, but in reality the only failure case is if a
pagefault occurs.  This is not a realistic problem as the kernel will almost
certainly crash with a double fault if this setup actually occured.

This is CVE-2016-9377 / part of XSA-196.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
master commit: 255e8fe95f22ded5186fd75244ffcfb9d5dbc855
master date: 2016-11-22 13:50:49 +0100

Index: xen-4.5.5-testing/xen/arch/x86/x86_emulate/x86_emulate.c
===================================================================
--- xen-4.5.5-testing.orig/xen/arch/x86/x86_emulate/x86_emulate.c
+++ xen-4.5.5-testing/xen/arch/x86/x86_emulate/x86_emulate.c
@@ -1452,10 +1452,16 @@ static int inject_swint(enum x86_swint_t
     {
         if ( !in_realmode(ctxt, ops) )
         {
-            unsigned int idte_size = (ctxt->addr_size == 64) ? 16 : 8;
-            unsigned int idte_offset = vector * idte_size;
+            unsigned int idte_size, idte_offset;
             struct segment_register idtr;
             uint32_t idte_ctl;
+            int lm = in_longmode(ctxt, ops);
+
+            if ( lm < 0 )
+                return X86EMUL_UNHANDLEABLE;
+
+            idte_size = lm ? 16 : 8;
+            idte_offset = vector * idte_size;
 
             /* icebp sets the External Event bit despite being an instruction. */
             error_code = (vector << 3) | ECODE_IDT |
@@ -1483,8 +1489,9 @@ static int inject_swint(enum x86_swint_t
              * Should strictly speaking read all 8/16 bytes of an entry,
              * but we currently only care about the dpl and present bits.
              */
-            ops->read(x86_seg_none, idtr.base + idte_offset + 4,
-                      &idte_ctl, sizeof(idte_ctl), ctxt);
+            if ( (rc = ops->read(x86_seg_none, idtr.base + idte_offset + 4,
+                                 &idte_ctl, sizeof(idte_ctl), ctxt)) )
+                goto done;
 
             /* Is this entry present? */
             if ( !(idte_ctl & (1u << 15)) )